1. Field of the Invention
The present invention relates to cementitious compositions such as ornamental and casting plasters, molding plasters, spackling compounds, and joint compounds for finishing joints between adjacent gypsum wallboards, and is more particularly concerned with a setting type of joint compound which may be prepared by pre-mixing with water and stored over reasonably long periods of time, and subsequently accelerated prior to application.
2. Description of the Prior Art
The use of drywall or wallboard construction, i.e. a non-plastered surface, has become increasingly popular over the years to the extent that it is estimated that a substantial amount of all new residential and commercial construction is finished in this manner. The usual drywall construction consists of gypsum wallboard panels secured to supporting framing members.
Gypsum wallboard is conventionally made by enclosing a core of an aqueous slurry of calcined gypsum between two paper layers. After the slurry has set and dried, the fabricated sheet is cut and the resulting panels are ready for use. It ordinarily is not practical to make wallboards over four feet wide and about eight to twelve feet in length. As a result there are a considerable number of joints between boards, and these joints must be reinforced and concealed for satisfactory appearance. According to one prior art practice batten strips were applied over the joints for this purpose. This was found to be unsatisfactory as the batten strips tended to accentuate the joints.
In order for wallboard use to be acceptable it is necessary to obtain a smooth finish wall construction similar to that obtained with conventional wet plaster methods. Accordingly, great effort has been made to conceal the joints between the drywall plasterboards or wallboards in order to achieve an overall smooth and continuous wall. To this end, cloth, metal and finally paper tapes have been cemented over the joints between the boards to reinforce the joints, and a finishing coat of cementitious material applied over the tapes to conceal the tape and to provide a smooth surface. The preferred conventional method is to recess the adjoining edges of the gypsum wallboard, apply an adhesive joint compound and embed the paper tape in the joint compound. Several finishing or feathering layers of the joint compound are applied to conceal the tape. Finally, the work is sanded to produce a smooth surface similar to the conventional wet plaster wall.
The adhesives or binding material commonly used in prior art joint compounds were generally based on proteins as the primary binders. Casein or certain refined soya proteins solubilized in water by alkaline materials were used extensively for this purpose. More recent materials are ethylene vinyl acetate and polyvinyl acetate emulsions. When properly blended with fillers such as ground asbestos, mica, clays, limestone, preservatives and thickening agents such as natural gums, a joint compound is obtained which is capable of binding the joint tapes in place and providing a smooth surface. However, this type of joint compound has the disadvantage of requiring extended periods of time to dry.
The conventional procedure for obtaining a good smooth finish is to apply the joint compound in several layers or coats. The first layer is primarily for the purpose of filling the space between the wallboards and for adhering the tape to the adjoining boards. The second layer is applied over the tape to form as smooth and as even a surface as is possible in order to provide a continuous surface coplanar with the board. However, due to shrinkage of the joint compound upon drying and irregularities due to trowelling, etc., a third or finishing layer is generally applied after the second coat has dried.
Due to the necessity for each layer to become substantially dry before the subsequent one can be applied, it is obvious that considerable periods of time are required to achieve an ultimate smooth drywall. Under certain environmental conditions, such as cold, damp weather, days or weeks may be required before the wall is completed. Such delays slow up the construction of a building and delay its eventual sale and occupancy.
Moreover, because of the slow drying of known adhesive type joint compounds shrinkage problems are magnified, contributing further to the difficulty of achieving a smooth wall surface. Shrinkage of the prior art joint compounds upon drying is especially serious when a second coat is applied over an earlier coat which is not completely dried. The areas of the earlier coating which are not thoroughly dry at the time of application of the second coat subsequently shrink, sometimes even after the wall is finished and decorated. Where delayed shrinkage is excessive, it necessitates a return to the job for refinishing and redecorating.
From the foregoing it is apparent that if a joint compound could be made which would set or harden quickly, not only would there be a substantial decrease in the time required for gypsum drywall construction, but, also, there would be less shrinkage. This would indeed represent a significant advance in the art and would greatly reduce the expense and time required for refinishing or redecorating the surfaces of a wall or ceiling.
In order to overcome the disadvantages of the drying type of joint compound, a setting type has been developed in the art. In this type of joint compound, calcined gypsum (calcium sulfate hemihydrate) is utilized as a substantial proportion of the filler material. The compound is prepared in dry powder form and mixed with water when ready for use. The water reacts with the calcium sulfate hemihydrate to form set gypsum or calcium sulfate dihydrate. In this compound the setting time is considerably shorter than the time required for the drying type to dry. This has considerably reduced the time required for preparing a suitable joint. However, the prior art setting type still has the disadvantage that it must be prepared and marketed in dry powder form since, when mixed, it has only a very short usable life and can not be prepared as a ready mixed material in liquid or slurry form as is possible with the drying type.